The invention pertains to a power generation plant comprising a combustion turbine and a compressed air energy system (CAES) with an air storage cavern and in particular to a means for the reduction of the pressure of air extracted from the storage cavern.
A typical power generation plant with a compressed air energy storage (CAES) system and a combustion as well as an air turbine is shown in FIG. 1. The CAES system comprises a cavern 1 underground, which stores air compressed by a compressor train 2 to a pressure of about 60 to 100 bar. A feed line 3 leads from the cavern 1 to a throttling valve 4, which reduces the air pressure to about 60 bar. The feed line 2 then leads to a heat exchanger or recuperator 5, where the air is preheated, and continues to a turbine train comprising an air turbine 6, a standard gas turbine 7, and a generator G. The air expands in the expansion air turbine 6 and exits through the turbine outlet at a pressure of approximately 15 bar. It then passes through a combustor 8 before entering the standard gas turbine 6. The exhaust gases from the standard gas turbine 7 are passed through a duct burner 9 and used to preheat the air in the recuperator 5 by known methods. At the outlet from the recuperator the exhaust gas pressure is about 1 bar.
The operation of this CAES system for power generation entails energy losses, which result in a loss in power that could have been generated by the plant. The energy losses occur in the throttling of the pressure of the cavern air in the form of heat generated in the valve 4. The air pressure reduction also represents a loss of some of the potential energy, which was put into the system by the compressor train 2 in order to store the air at cavern pressure. Such losses could be decreased by a reduction of the cavern pressure either by decreasing the mass of the air stored in the cavern or by increasing the volume of the cavern. However, a decrease in stored air mass would result in a shortening of the possible plant operation time using the stored air to supplement the air provided by the compressor for the combustion turbine. Furthermore, an increase in the cavern volume is rarely an economically feasible solution.
A CAES system can also be implemented in power generation plants with a standard gas turbine and a combustion turbine compressor. The CAES system is operated in order to supplement the compressed air, which is normally provided by the combustion turbine compressor. Such a gas turbine operated with a supplemental compressed air supply is disclosed in WO 00/01934. Referring to FIG. 2 of that disclosure, such a power generation plant comprises a combustor C, a gas turbine 16, a compressor 14, and generator G, which are connected to a system 10 that provides supplemental air during periods of peak power demand. The air supply system 10 comprises an air cavern 28, a compressor train 32 for compressing the air to be stored, throttling valves 54, 55, 56, which reduce the pressure of the air extracted from the cavern 28. The air is led, via a recuperator 52, to the combustor C and gas turbine 16.
The pressure of the air extracted from the cavern must be matched to a pressure suitable for the combustor, which is typically in the range between 10 and 20 bars. For this, the relatively high cavern air pressure of 60 to 100 bars is throttled by means of valve 54. The throttling, however, causes an overall power loss in the power generation system. Such throttling losses could be decreased by methods as described above in connection with FIG. 1. Again however, neither of these methods is an economically feasible solution.
A power generation plant with a CAES system requires the pressure of the air when it is extracted from the storage cavern to be reduced to 10-20 bar, which is a suitable air pressure for the operation of a combustion turbine. The air pressure in the cavern varies greatly within a range from 60 to 100 bars for example. This pressure range is of much greater size than is typically encountered in a standard power generation plant. In a standard steam turbine power plant, for example, the pressure from the boiler remains within a small range and the steam turbine is designed for that particular small pressure range. In a standard gas turbine power plant operating with a combustion turbine compressor only, the pressure of the air compressed by the compressor is within a small range that is suitable for the operation of the combustion turbine. Hence, pressure reduction schemes found in standard steam and combustion turbine systems are not readily applicable to a power generation plant with compressed air storage or supplemental air as discussed in this background art.
In view of the described background art, it is an object of the invention to provide a power generation plant having a compressed air energy storage system with a means to reduce the pressure of air extracted from a compressed air storage cavern with varying pressure for the use in a turbine or turbine train. In particular, the pressure of the extracted air must be reduced to a level suitable for the operation of a combustion turbine. In view of the background cited, the means for pressure reduction should avoid power losses and provide an increased efficiency of the power generation plant as a whole.
A power generation plant comprises a compressed air energy system with a cavern for the storage of compressed air, at least one combustion turbine or a turbine train with at least one air turbine and at least one combustion turbine, which drive a generator on a rotor shaft, and means to reduce the pressure of the air that is extracted from the storage cavern for the use in the turbine train or combustion turbine.
According to the invention, the means for reducing the pressure of air extracted from the storage cavern comprises at least one expansion turbine, which is arranged on a rotor shaft that drives a generator. This at least one expansion turbines is additional to the combustion turbine or turbine train. Furthermore, the expansion turbine has means to control the size of the pressure reduction.
The means for pressure reduction by an expansion turbine enables primarily a pressure reduction of the size necessary (from a range from 100 bars to 60 bars down to about 20 bars) in a power plant with a CAES system or a standard turbine operated with supplemental compressed air stored in a cavern. The power generation plant with this type of pressure reduction has the advantage over plants of the state of the art in that it avoids the energy losses caused by conventional throttling valves. Instead, the energy, which otherwise is lost in the process of pressure reduction, directly drives the generator. Furthermore, the pressure reduction system according to the invention allows the storage of the same mass of air at a same pressure as in systems of the state of the art. This in turn allows an operation of the power generation plant using compressed air from the cavern over a period of time that is not limited by a reduction of mass or pressure of the cavern air. Furthermore, a cavern of same size may be used.
In a first embodiment of the invention, the means for reducing the pressure of the air extracted from the cavern comprises several additional expansion turbines arranged preceding the turbine train or at least one combustion turbine. Several inlet lines are arranged to connect a feed line from the air cavern to each of the expansion turbines, each inlet line having a valve. Different settings of the valves allow a pressure reduction of various size, as for example by an expansion in just one expansion turbine, in some, or all the expansion turbines consecutively.
In a second embodiment of the invention, the means for pressure reduction comprises one single expansion turbine having several extraction lines leading to the turbine train or combustion train, each extraction line having a valve. The several extraction lines with a valve allow an air pressure reduction by an expansion in the entire expansion turbine or in a section of various size of the turbine.
In a third embodiment of the invention, the means for pressure reduction comprises one single expansion turbine having several inlet lines leading from a feed line from the storage cavern to different sections of the turbine, each inlet line having a valve. According to the setting of the valve in each inlet line, the extent of air expansion and size of pressure reduction can be varied.
The means for pressure reduction according to the several embodiments of the invention described above are, in a first variant, arranged on the rotor shaft of the combustion turbine of the power generation plant, where in particular, the rotor shaft of the one or several additional expansion turbines are disengageable from the rotor shaft of the combustion turbine.
In a second variant of the several embodiments of the invention, the one or several additional expansion turbine for pressure reduction are arranged on the rotor shaft of the combustion turbine, where in particular, the rotor shaft of the expansion turbine is permanently engaged with the rotor shaft of the combustion turbine.
In a third variant of the several embodiments of the invention, the one or several additional expansion turbines of the pressure reduction system are arranged on a rotor shaft that is separate from the rotor shaft of the combustion turbine and that drives an additional generator.
In the various embodiments of the invention mentioned above, the air from the cavern is led to the expansion turbine, in which case it operates as an expansion air turbine.
In a further embodiment of the invention, a combustor is arranged preceding the additional expansion turbine. The air from the cavern is first led through the combustor and then on to the expansion turbine, in which case it operates as a combustion turbine.
In yet further embodiments of the invention, the air from the cavern is led either directly, or via a heat exchange apparatus of any kind, such as a recuperator, to the expansion turbine. In both cases the expansion turbine is either an expansion air turbine or a combustion turbine.
The various embodiments and variants of the invention can be implemented in a power generation plant with a typical CAES system, where the generator is driven by an air turbine and a combustion turbine.
They can also be implemented in a power generation plant comprising a standard combustion turbine with a combustion turbine compressor and a CAES system that provides supplemental compressed air from a storage cavern during times of high power demand.